Aluminum alloy fin material for heat exchanger use excellent in buckling resistance and method for manufacturing same

ABSTRACT

An aluminum alloy fin material for heat exchanger use excellent in buckling resistance which, even if the fin material is reduced to a final sheet thickness of 30 to 80 μm, is excellent in formability at the time of corrugation, has a suitable strength before brazing enabling easy fin formation, exhibits a suitable self corrosion resistance and sacrificial anode effect, and is high in strength as a material for forming working fluid passages is provided. By mass %, Si: 1.3% to 1.6%, Fe: 0.30% to 0.70%, Mn: 1.8% to 2.3%, Zn: 0.5% to 2.0%, and Ti: 0.002% to 0.10% are contained, as impurities, Mg is limited to 0.05% or less and Cu is limited to 0.06% or less, and the balance is unavoidable impurities and Al, a final sheet thickness is 30 to 80 μm, a tensile strength is 260 MPa or less, a solidus temperature is 615° C. or more, and further a tensile strength when measured after brazing heating then cooling, is 170 MPa or more and a spontaneous potential is −780 mV to −700 mV.

FIELD

In an aluminum heat exchanger, as a material for forming the aluminum working fluid passages etc., an aluminum alloy fin material which has been brazed is used. To improve the characteristics of a heat exchanger in performance, in this aluminum alloy fin material, a sacrificial anode effect is demanded for preventing corrosion of the material forming the working fluid passages. Along with this, an excellent sag resistance and erosion resistance are demanded so that the material does not deform due to the high temperature heating at the time of brazing or the solder does not penetrate through it.

Along with a lighter weight of the heat exchanger, reduction of the thickness of the fin material forming the working fluid passages is demanded. As the material has been made increasingly thinner, since the fin material for heat exchanger use is brazed after being attached to other heat exchanger use members, the strength before brazing heating and the high temperature buckling resistance have also been studied.

PTL 1 proposes a fin material made of aluminum alloy for heat exchanger use having high strength and high heat resistance characterized by containing Mn: 0.8 to 2.0% (wt %, same below), Si: 0.2 to 0.6%, and Zn: 0.4 to 2.0%, restricting Cu to 0.03% or less and Fe to 0.2% or less, and having a balance of Al and unavoidable impurities and, further, containing 0.02 to 0.3 μm in range of size of intermetallic compounds in 600/μm³ or more, 3 μm or more size of intermetallic compounds being restricted to 500/mm² or less, an average crystal grain size of a front surface after brazing heating being 0.4 mm or more, and further having a sheet thickness of 0.03 to 0.10 mm in range and a tensile strength of 200 N/mm² or more.

Even if the fin material for heat exchange use proposed in PTL 1 is high in strength before brazing (original sheet strength) and has a sheet thickness of a thin 0.1 mm or less, there is very little liability of the material deforming or buckling at the time of assembly of the heat exchanger. Further, the high temperature buckling resistance is also excellent and there is little liability of buckling due to the high temperature at the time of brazing.

Further, the fin material for heat exchanger use is corrugated etc. to be formed into a predetermined shape before brazing the fin material and other members for heat exchanger use, so the demands on formability have become stricter. Furthermore, the fin material has Mn, Fe, Si, Zn, etc. added to it so as to satisfy the above such basic characteristics, so recently the manufacturing process has been improved, therefore a high strength aluminum alloy fin for heat exchanger use low in tensile strength before brazing and high in tensile strength and heat conductivity after brazing has been developed.

PTL 2 proposes a method for manufacturing a high strength aluminum alloy fin material for heat exchanger use characterized by continuously casting a melt containing Si: 0.5 to 1.4 wt %, Fe: 0.15 to 1.0 wt %, Mn: 0.8 to 3.0 wt %, and Zn: 0.5 to 2.5 wt %, further limiting the Mg as an impurity to 0.05 wt % or less, and having a balance of usual impurities and Al by a twin belt casting machine to a thickness 5 to 10 mm thin slab, taking it up into a roll, then cold rolling to a sheet thickness of 0.08 to 2.0 mm, process annealing it at a holding temperature of 350 to 500° C., and cold rolling by a cold rolling rate of 50 to 96% to a final sheet thickness of 40 μm to 200 μm.

The fin material proposed in PTL 2 has a high strength and heat conductivity after brazing and is excellent in sag resistance, erosion resistance, self corrosion resistance, and sacrificial anode effect.

As explained above, the fin material for heat exchanger use is formed into a predetermined shape by corrugation etc. before brazing the fin material and other members for heat exchanger use. At this time, there was the problem that the shaping dies are increasingly worn by second phase particles of a high hardness present in the metal structure of the fin material and the die set becomes shorter in lifetime.

PTL 3 discloses the art of improving the die wear property by prescribing the number of 1 μm or more second phase particles per unit area present in the metal structure of the fin material. Specifically, PTL 3 proposes an aluminum alloy fin material for heat exchanger use high in strength and excellent in heat transmission property, sacrificial corrosion property, brazing property, and wear resistance of dies characterized by having a chemical composition comprising, by mass %, Si: 0.8 to 1.0%, Fe: 1.1 to 1.4%, Mn: 0.6 to 0.7%, and Zn: 0.5 to 0.9%, where Fe+Mn: 2.0% or less, and a balance of Al and unavoidable impurities, where Mg as an unavoidable impurity is 0.05% or less, a solidus temperature is 620° C. or more, and, in a state after brazing, a yield strength is 40 MPa or more, a conductivity is 49.5% IACS or more, a spontaneous potential is −740 mV or less, and a number of second phase particles of a circle equivalent diameter of 1 μm or more per unit area observed in the metal structure is 6000/mm² or less.

In the fin material proposed in PTL 3, the particle density of the second phase particles with a high hardness present in the metal structure of the fin material is controlled to improve the die wear property, but the contents of the Si and Mn in the fin material are small, so if trying to make the fin material thinner, there is a concern about the fin material used as the material for forming the working fluid after brazing becoming easier to buckle.

PTL 4 proposes a fin material containing, by mass %, Si: 0.9 to 1.2%, Fe: 0.8 to 1.1%, Mn: 1.1 to 1.4%, and Zn: 0.9 to 1.1%, limiting, as impurities, Mg to 0.05% or less, Cu to 0.03% or less, and ([Si]+[Fe]+2[Mn])/3 to 1.4% to 1.6%, and having a balance of unavoidable impurities and Al, where a sheet thickness is 35 to 50 μm, a tensile strength before brazing is 215 MPa or less, a solidus temperature is 620° C. or more, a tensile strength after brazing is 140 MPa or more, a conductivity after brazing is 45% IACS or more, and a spontaneous potential after brazing is−730 mV to −760 mV.

According to the fin material proposed in PTL 4, it is possible to obtain an aluminum alloy fin material for heat exchanger use which is small in amount of springback, has a suitable strength before brazing enabling easy fin formation, further has a high strength after brazing, and is excellent in die wear property, erosion resistance, self corrosion resistance, and sacrificial anode effect.

In the fin material proposed in PTL 4, the amount of springback is small and the material has a suitable strength before brazing enabling easy fin formation, but in a specific composition, ([Si]+[Fe]+2[Mn])/3 is limited to 1.4% to 1.6%, so if trying to make the fin material thinner, there is a concern about the fin material used as working fluid members after brazing becoming easier to buckle.

CITATIONS LIST Patent Literature

-   [PTL 1] Japanese Unexamined Patent Publication No. 2000-119783 -   [PTL 2] Japanese Unexamined Patent Publication No. 2005-002383 -   [PTL 3] Japanese Unexamined Patent Publication No. 2009-270180 -   [PTL 4] Japanese Unexamined Patent Publication No. 2014-047384

Therefore, it is desirable to prescribe the alloy composition in a suitable range and, as the method for manufacture, cast a slab by a twin-belt casting machine and suitably combine cold rolling and annealing under prescribed conditions to thereby obtain a fin material for heat exchanger use excellent in formability at the time of corrugation even if the material is reduced to a final sheet thickness of 30 to 80 μm, having a suitable strength before brazing enabling easy fin formation, exhibiting a suitable self corrosion resistance and sacrificial anode effect, and high in strength and excellent in buckling resistance as a material for forming working fluid passages.

SUMMARY Technical Problem

An object of the present invention is to provide an aluminum alloy fin material for heat exchanger use excellent in buckling resistance comprised of a fin material reduced to a final sheet thickness of 30 to 80 μm which is excellent in formability at the time of corrugation, has a suitable strength before brazing enabling easy fin formation, exhibits a suitable self corrosion resistance and sacrificial anode effect, and is high in strength as a material forming working fluid passages.

Solution to Problem

To achieve this object, the fin material of the present invention (first aspect of the invention of the present application) provides an aluminum alloy fin material for heat exchanger use containing, by mass %, Si: 1.3% to 1.6%, Fe: 0.30% to 0.70%, Mn: 1.8% to 2.3%, Zn: 0.5% to 2.0%, and Ti: 0.002% to 0.10%, further limiting, as impurities, Mg to 0.05% or less and Cu to 0.06% or less, and having a balance of unavoidable impurities and Al, where a final sheet thickness is 30 to 80 μm, a tensile strength is 260 MPa or less, a solidus temperature is 615° C. or more, and further a tensile strength when measured after brazing heating then cooling, is 170 MPa or more and a spontaneous potential is −780 mV to −700 mV.

The method for manufacturing a fin material of the present invention (second aspect of the invention of present application) comprises a continuous casting step of pouring a melt of the above composition and using a twin-belt casting machine to continuously cast then take up in a coil a thickness 6 to 15 mm slab, a primary cold rolling step of cold rolling to a sheet thickness of 1.0 to 6.0 mm, a primary process annealing step of process annealing at 360 to 460° C., a secondary cold rolling step of cold rolling to a sheet thickness of 0.05 to 0.12 mm, a secondary process annealing step of process annealing at 200 to 350° C., and a final cold rolling step of cold rolling by a cold rolling rate of 20 to 50% to a final sheet thickness of 30 to 80 μm.

Advantageous Effects of Invention

According to the first aspect of the invention of the present application, it is possible to provide an aluminum alloy fin material for heat exchanger use excellent in buckling resistance which is excellent in formability at the time of corrugation even if the final sheet thickness is reduced to 30 to 80 μm, has a suitable strength before brazing enabling easy fin formation, exhibits a suitable self corrosion resistance and sacrificial anode effect, and is high in strength as a material forming working fluid passages.

According to the second aspect of the invention of the present application, it is possible to use an aluminum melt prescribed in composition so as to form a slab by a twin-belt casting machine and to suitably combine cold rolling and annealing under prescribed conditions to thereby manufacture an aluminum alloy fin material for heat exchanger use provided with the above prescribed conditions.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing an outer appearance of a brazing test piece.

FIG. 2 is a graph showing a relationship between a load (N) and displacement (mm) when using a brazing test piece shown in FIG. 1 for a compression test.

DESCRIPTION OF EMBODIMENTS

Aluminum Alloy Fin Material for Heat Exchanger Use Below, the reasons for limitation of the composition of the first aspect of the invention of the present application of an aluminum alloy fin material for heat exchanger use and the final sheet thickness, tensile strength, solidus temperature, tensile strength after brazing heating, and spontaneous potential after brazing heating will be explained. In the Description of the present application, unless otherwise indicated, “%” shall mean “mass %”.

Si: 1.3% to 1.6%

Si, in the copresence of Fe and Mn, forms a submicron level Al—(Fe.Mn)—Si-based compound at the time of brazing and has the effect of enhancing the tensile strength after brazing and the buckling strength. With a content of Si of less than 1.3%, the effect is not sufficient, while if over 1.6%, the solidus temperature falls, so at the time of brazing, erosion is liable to be caused in the fin material. Therefore, the Si content is 1.3% to 1.6% in range. The preferable Si content is 1.4% to 1.55% in range. The more preferable Si content is over 1.4% to 1.5% in range.

Fe: 0.30% to 0.70%

Fe, in the copresence of Mn and Si, forms a submicron level Al—(Fe.Mn)—Si-based compound at the time of brazing, contributes to dispersion strengthening, and enhances the strength after brazing. To obtain this effect, the Fe content has to be 0.30% or more. If the Fe content is less than 0.30%, the strength falls. On the other hand, if the Fe content is over 0.70%, at the time of casting of the alloy, coarse Al—(Fe.Mn)—Si-based crystallites form and manufacture of the sheet material becomes difficult. Therefore, the Fe content is 0.30% to 0.70% in range. The preferable Fe content is 0.30% to 0.65% in range. The more preferable Fe content is 0.30% to 0.60% in range.

Mn: 1.8% to 2.3%

Mn, by causing the copresence of Fe and Si, precipitates at a high density as a submicron level Al—(Fe.Mn)—Si-based compound at the time of brazing and has the effect of enhancing the tensile strength after brazing and the buckling strength. Further, submicron level Al—(Fe.Mn)—Si-based precipitates have a strong recrystallization inhibiting action, so the recrystallized grains become 200 μm or more, the erosion resistance can be secured, and the brazeabiliity is improved. To obtain this effect, the Mn content has to be 1.8% or more. If the Mn content is over 2.3%, the tensile strength before brazing becomes too high and the formability falls. Therefore, the Mn content is 1.8% to 2.3% in range. The preferable Mn content is 1.9% to 2.3% in range. The more preferable Mn content is over 2.0% to 2.3% in range.

Zn: 0.5% to 2.0%

Zn makes the spontaneous potential after brazing the fin material a base potential, so has a sacrificial anode effect. To obtain this effect, the Zn content has to be 0.5% or more. However, if the Zn content is over 2.0%, the spontaneous potential become excessively base and the self corrosion resistance of the material deteriorates. Therefore, the Zn content is 0.5% to 2.0% in range. The preferable Zn content is 0.5% to 1.9% in range. The more preferable Zn content is 0.5% to 1.8% in range.

Ti: 0.002% to 0.10%

Ti acts as a crystal grain refining agent at the time of slab casting and prevents cracking of the slab at the time of casting. To obtain this effect, the Ti content has to be 0.002% or more. However, if the Ti content is over 0.10%, coarse intermetallic compounds TiAl₃ precipitate at the time of casting. During the cold rolling, the TiAl₃ form starting points liable to cause defects. Therefore, the Ti content is 0.002% to 0.10% in range. The preferable Ti content is 0.002% to 0.07% in range. The more preferable Ti content is 0.005% to 0.05% in range.

Mg: 0.05% or Less

Mg affects the brazeability. If the content is over 0.05%, the brazeability is liable to be impaired. In particular, in the case of brazing using a fluoride-based flux, the fluorine (F) in the constituents of the flux and the Mg in the alloy easily react whereby MgF₂ or other compounds are formed. For this reason, the absolute amount of the flux effectively acting at the time of brazing becomes insufficient and brazing defects easily are caused. Therefore, in the unavoidable impurities, in particular the content of Mg is limited to 0.05% or less.

Cu: 0.06% or Less

Cu makes the potential precious to improve the tensile strength after brazing, but if over 0.06%, the tensile strength before brazing becomes too high and the formability is liable to be made to deteriorate. Therefore, in the unavoidable impurities, in particular the content of Cu is limited to 0.06% or less.

Other Unavoidable Impurities

Unavoidable impurities are elements which unavoidably enter from the raw material base metal, recycled material, etc. Among these elements, in particular Cr, Zr, and Ni cause the heat conductivity (electric conductivity) to fall even in trace amounts, so these are respectively restricted to less than 0.05 mass %. If B increases in content, while depending on the Ti content as well, the effect of refining the crystal grains is liable to fall, so the content is restricted to less than 0.03%. The allowable contents of these other non-controlled elements (for example, Pb, Bi, Sn, Na, Ca, and Sr) are respectively restricted to less than 0.02%. The others (for example, Ga, V, Co, Nb, Mo, and W) are respectively restricted to less than 0.05%. Note that even if containing non-controlled elements in these ranges, the advantageous effect of the present invention is not obstructed.

Final Sheet Thickness of 30 to 80 μm

To reduce the thickness and lighten the weight, the final sheet thickness is limited to 80 μm or less. Further, with a final sheet thickness of less than 30 μm, insufficient strength of the heat exchanger itself after brazing the fins is invited. Therefore, the final sheet thickness of the fin material is restricted to 30 to 80 μm.

Tensile Strength of 260 MPa or Less

If the tensile strength before brazing is over 260 MPa, in the case of a sheet thickness 30 to 80 μm thin fin material, the formability at the time of fin formation is liable to fall and the predetermined shape is liable to be unable to be obtained, so this is not preferable. Therefore, the tensile strength before brazing is restricted to 260 MPa or less.

Solidus Temperature of 615° C. or More

If the solidus temperature is less than 615° C., the possibility of erosion occurring at the time of brazing increases, so this is not preferable. Therefore, the solidus temperature is limited to 615° C. or more.

Tensile Strength When Measured After Brazing Heating and Then Cooling is 170 MPa or More

The first aspect of the invention of the present application of the fin material is brazed to a tube etc. and used as a heat exchanger. For this reason, the heat exchanger as a whole has to satisfy the predetermined demanded strength. The higher the buckling load of a fin used as a working fluid component after brazing the more preferable. Therefore, the tensile strength after brazing heating is limited to 170 MPa or more.

Spontaneous Potential When Measured After Brazing Heating and Then Cooling is −780 mV to −700 mV

The spontaneous potential of the first aspect of the invention of the present application of the fin material means the potential based on a silver-silver chloride electrode (SSE: Ag/AgCl/5% NaCl aqueous solution). If the spontaneous potential after brazing is less than −780 mV, the potential becomes too base and the fin material falls in self corrosion resistance, so this is not preferable. If the spontaneous potential after brazing is over −700 mV, the potential becomes too precious and the fin material falls in sacrificial anode effect, so this is not preferable. Therefore, the spontaneous potential after brazing heating is −780 mV to −700 mV in range. The preferable spontaneous potential after brazing heating is −760 mV to −700 mV in range. The more preferable spontaneous potential after brazing heating is −750 mV to −700 mV in range.

Method for Manufacturing Aluminum Alloy Fin Material for Heat Exchanger Use

Next, casting conditions of a slab and manufacturing conditions etc. of the primary cold rolling step, primary process annealing step, secondary cold rolling step, secondary process annealing step, and final cold rolling step for a second aspect of the invention of the present application of an aluminum alloy fin material for heat exchanger use will be explained.

Slab Casting Step

Use of Twin-Belt Casting Machine

The twin-belt casting machine is provided with a pair of facing rotating belt parts provided with endless belts, a cavity formed between the pair of rotating belt parts, and a cooling means provided at the inside of the rotating belt parts. A machine which supplies a metal melt through a nozzle made of refractories to the inside of the cavity to continuously cast a slab etc. can be used. By using such a twin-belt casting machine, a facing step, homogenization treatment step, and hot rolling step can be omitted.

Slab Thickness Made 6 to 15 mm

In the second aspect of the invention of the present application, the thickness of the slab cast is limited to 6 to 15 mm. With this thickness, if the solidification speed at the center of the sheet thickness is fast and the structure is uniform and further the composition is in the scope of the present invention, it is possible to obtain a fin material having various excellent properties with few coarse compounds and large crystal grain sizes after brazing. If the slab thickness is less than 6 mm, the amount of aluminum passing through the twin-belt casting machine per unit time becomes too small and casting becomes difficult. If the thickness is over 15 mm, it is difficult to coil up the cast slab as it is. Accordingly, the slab thickness is limited to 6 to 15 mm.

If using a twin-belt casting machine to cast a thickness 6 to 15 mm slab, the cooling speed at the position of slab ¼ thickness is 20 to 200° C./sec or so. By the melt cooling by a relatively fast cooling speed in this way, it becomes possible to suppress precipitation of Al—(Fe.Mn)—Si and other coarse intermetallic compounds at the time of casting in the range of chemical composition of the present invention and raise the amount of solid solution of Fe, Si, Mn, and other elements in the matrix.

In the second aspect of the invention of the present application, the slab thickness is 6 to 15 mm. The slab can be coiled up as it is, but for example it is also possible to use a hot rolling mill for skin pass rolling of a rolling reduction of 5 to 10% or so. By doing this, it is possible to improve the flatness of the surface and the surface quality of the coil is improved.

Primary Cold Rolling Step In the primary cold rolling step, the coiled up slab is cold rolled by several passes by a cold rolling mill to obtain a sheet thickness of 1.0 to 6.0 mm. If over a sheet thickness of 6.0 mm, the cold rolling rate is too low and is unsuitable. If less than a sheet thickness of 1.0 mm, the work hardening becomes too vigorous and it is difficult to continue cold rolling.

Primary Process Annealing Step The holding temperature of the primary process annealing is made 360 to 460° C. If the holding temperature of the primary process annealing is less than 360° C., a sufficiently softened state cannot be obtained. If the holding temperature of the primary process annealing is over 460° C., the Al—(Fe.Mn)—Si-based compound precipitating in the matrix at the time of process annealing becomes coarse, so the effect of dispersion strengthening becomes smaller and the desired tensile strength after brazing is liable to be unable to be obtained, so this is not preferable. The holding temperature of the primary process annealing is preferably 380 to 440° C.

The holding time of the primary process annealing does not have to be particularly limited, but making it 1 to 5 hr in range is preferable. With a holding time of the primary process annealing of less than 1 hr, if the temperature of the coil as a whole remains uneven, there is a possibility of a uniformly softened state in the sheet not being obtained, so this is not preferable. If the holding time of the primary process annealing is over 5 hr, too much time is taken for the treatment and the productivity falls, so this is not preferable.

The speed of temperature rise and the cooling speed at the time of the primary process annealing treatment do not have to be particularly limited, but making them 30° C./hr or more is preferable. If the speed of temperature rise and the cooling speed at the time of the primary process annealing treatment are less than 30° C./hr, too much time is taken for treatment and the productivity falls, so this is not preferable.

Secondary Cold Rolling Step

In the secondary cold rolling step, the sheet treated by primary process annealing is cold rolled for several passes by a cold rolling mill to reduce the sheet thickness to 0.05 to 0.12 mm. If the sheet thickness is over 0.12 mm, the amount of strain accumulated in the sheet due to the cold rolling is too small and it is not possible to obtain a sheet having a predetermined metal structure after the secondary process annealing step. If the sheet thickness is less than 0.05 mm, it becomes difficult to perform the final cold rolling step with a final cold rolling rate of 20 to 50% after the secondary process annealing step and becomes difficult to obtain a predetermined tempered fin material.

Secondary Process Annealing Step The holding temperature of the secondary process annealing is made 200 to 350° C. If the holding temperature of the secondary process annealing is less than 200° C., it is not possible to obtain a sufficiently softened state. As opposed to this, if the holding temperature of the secondary process annealing is over 350° C., the Al—(Fe.Mn)—Si-based compound precipitating in the matrix ends up becoming coarser, so the effect of dispersion strengthening become smaller and the desired tensile strength after brazing is liable to be unable to be obtained. The holding temperature of the secondary process annealing is preferably made 220 to 330° C.

The holding time of the secondary process annealing is not particularly limited, but making it 1 to 5 hr in range is preferable. With a holding time of the secondary process annealing of less than 1 hr, there is a possibility of the temperature of the coil as a whole remaining uneven and sufficient softening not being able to be obtained, so this is not preferable. If the holding time of the secondary process annealing is over 5 hr, the treatment takes too much time and the productivity falls, so this is not preferable.

The speed of temperature rise and the cooling speed at the time of secondary process annealing treatment are not particularly limited, but making them 30° C./hr or more is preferable. If the speed of temperature rise and the cooling speed at the time of secondary process annealing treatment are less than 30° C./hr, the treatment takes up too much time and the productivity falls, so this is not preferable.

Final Cold Rolling Step With Final Cold Rolling Rate of 20 to 50% In the final cold rolling step, the final cold rolling rate is limited to 20 to 50%. If the final cold rolling rate is less than 20%, there is little strain energy accumulated by the cold rolling and recrystallization is not completed in the process of rise of temperature at the time of brazing, so the sag resistance and erosion resistance fall. If the final cold rolling rate exceeds 50%, the product strength becomes too high and it becomes difficult to obtain a predetermined fin shape in the fin formation.

The thus manufactured aluminum alloy fin material (final sheet thickness of 30 to 80 μm) is slit into predetermined widths, then corrugated and alternately stacked with a flat tube comprised of a material for working fluid passage use, for example, a clad sheet made of 3003 alloy etc. covered with a brazing material and brazed to thereby form a heat exchanger unit.

EXAMPLES Example A

In this Example A, the results of study of the chemical composition of the first aspect of the invention of the present application of the aluminum alloy fin material for heat exchanger use will be explained.

Each alloy melt of 11 levels of compositions was melted in a #10 crucible. A small-sized lance was used to blow in inert gas for 5 minutes for degassing. A sample for analysis use was taken in a disk shape, then the alloy melt was cast into an inside dimension 200×200×16 mm water-cooled die to fabricate a slab. The analyzed values of the compositions (chemical compositions) of the materials (11 levels) obtained by emission spectroscopy are shown in Table 1.

TABLE 1 Chemical Composition of Sample Materials (mass %) Alloy no. Si Fe Cu Mn Zn Mg Ti Al Ex. 1 1 1.41 0.60 0.02 2.00 0.76 <0.01 0.01 bal. Ex. 2 2 1.41 0.50 0.05 2.02 1.09 <0.01 0.01 bal. Ex. 3 3 1.37 0.69 0.06 2.01 0.81 <0.01 0.01 bal. Comp. Ex. 1 4 1.27 0.65 0.02 1.94 1.03 <0.01 0.006 bal. Comp. Ex. 2 5 1.07 0.50 0.02 1.67 1.49 <0.01 0.01 bal. Comp. Ex. 3 6 1.13 0.98 0.02 1.58 0.95 <0.01 0.01 bal. Comp. Ex. 4 7 1.14 0.98 0.05 1.58 1.30 <0.01 0.01 bal. Comp. Ex. 5 8 1.12 0.41 0.07 1.99 1.21 <0.01 0.01 bal. Comp. Ex. 6 9 1.41 0.41 0.02 1.62 1.19 <0.01 0.01 bal. Comp. Ex. 7 10 1.02 0.51 0.02 1.53 0.75 <0.01 0.004 bal. Comp. Ex. 8 11 1.19 0.85 0.02 1.69 1.03 <0.01 0.006 bal. Note) Underlined values show values outside prescribed range of present application.

Each of these slabs was shaved at its two surfaces by 3 mm each, then a first stage of cold rolling was performed to a sheet thickness of 4.0 mm, the temperature inside the annealing furnace was raised by a speed of temperature rise of 50° C./hr, the temperature was held at 400° C.×2 hr, then the sheet was air cooled as primary process annealing treatment. Further, a second stage of cold rolling was performed to a sheet thickness of 0.083 mm, the temperature inside the annealing furnace was raised by a speed of temperature rise of 50° C./hr, the temperature was held at 300° C.×2 hr, then the sheet was air cooled as secondary process annealing treatment. Further, the sheet was cold rolled by a final cold rolling rate of 40% to obtain a fin material with a final sheet thickness of 50 μm (tempering symbol: H₁ ₄).

The fin materials of the compositions of the above obtained Alloy No. 1 to Alloy No. 11 were tested and measured as in the following (1) to (3).

(1) Properties Before Brazing Heating

Test Item

[1] Tensile strength (MPa)

As the tensile strength before brazing heating, the tensile strengths of the fin materials of the compositions of the Alloy No. 1 to Alloy No. 11 obtained were measured. The results of measurement of the tensile strengths obtained are shown in Table 2.

[2] Solidus temperature (° C.)

The fin materials of the compositions of the Alloy No. 1 to Alloy No. 11 obtained were measured for solidus temperatures by differential thermal analysis. The results of measurement of the solidus temperatures obtained are shown in Table 2.

(2) Properties After Brazing Heating

Brazing Heating Conditions

Envisioning actual brazing heating conditions, the fin materials of the compositions of the Alloy No. 1 to Alloy No. 11 obtained were raised in temperature from room temperature to near 600° C. by an average speed of temperature rise of 50° C./min, the temperature was held near 600° C. for 2 minutes or so, then the sheets were cooled by an average cooling speed of 100° C./min as heat treatment. That is, the fin materials of the compositions of the Alloy No. 1 to Alloy No. 11 obtained were heat treated as above without using a brazing material.

Test Item

[1] Tensile strength (MPa)

As the tensile strength after brazing heating, the tensile strengths of the fin materials of the compositions of the Alloy No. 1 to Alloy No. 11 after heat treatment were measured. The results of measurement of the tensile strengths obtained are shown in Table 2.

[2] Spontaneous potential (mV)

As the spontaneous potential after brazing heating, the fin materials of the compositions of the Alloy No. 1 to Alloy No. 11 after the above heat treatment were measured for spontaneous potential (mV) after immersion in a 5% saline solution for 60 minutes using silver-silver chloride electrodes (saturated). The results of measurement of the spontaneous potentials obtained are shown in Table 2.

(3) Measurement of Buckling Load

The fin materials of the compositions of the Alloy No. 1 to Alloy No. 11 obtained above (total 11 levels) were worked into corrugated shapes of a height of 2.3 mm×width of 21 mm×pitch of 3.4 mm (8 peaks) (three prepared for each level). Next, thickness 0.25 mm brazing sheets (brazing material 4045 alloy, clad rate 8%) coated with noncorrosive fluoride-based flux were prepared. The brazing sheets were set above and below the corrugated fin materials. The assemblies were raised in temperature from room temperature to near 600° C. by an average speed of temperature rise of 50° C./min, were held near 600° C. for 2 minutes or so, then were cooled by an average cooling speed of 100° C./min to thereby prepare brazing test pieces. The appearance of the brazing test pieces is shown in FIG. 1. These brazing test pieces were used for a compression tests and, as shown in FIG. 2, were measured for maximum loads at that time. These were defined as the buckling loads. The test was conducted three times for each level and the average value of the buckling load measured three times was calculated. The average value was used for evaluation. The obtained results of measurement of the buckling loads and average values of the same (n=3) are shown in Table 2.

TABLE 2 Properties of Test Materials (Study of Chemical Composition) Before brazing heating After brazing heating Solidus Tensile Tensile Spontaneous Alloy temperature strength strength potential Buckling load (N) no. (° C.) (MPa) (MPa) (mV) 1 2 3 Average value Ex. 1 1 620 244 173 −731 294 322 272 296 Ex. 2 2 618 256 174 −736 284 305 308 299 Ex. 3 3 619 259 177 −709 269 296 325 297 Comp. Ex. 1 4 623 243 169 −758 289 239 278 269 Comp. Ex. 2 5 624 238 160 −797 226 292 301 273 Comp. Ex. 3 6 626 238 163 −763 271 264 244 260 Comp. Ex. 4 7 625 248 165 −763 246 240 242 243 Comp. Ex. 5 8 626 262 167 −746 311 302 292 302 Comp. Ex. 6 9 611 226 168 −753 279 282 305 289 Comp. Ex. 7 10 627 231 152 −742 273 245 262 260 Comp. Ex. 8 11 626 230 161 −764 285 249 297 277 Note) Underlined values show values outside prescribed range of present application.

The fin materials of Examples 1 to 3 (Alloy Nos. 1 to 3) were within the scope of composition of the present invention, so in each case, the solidus temperature was 615° C. or more and the brazeability (erosion resistance) was excellent, the tensile strength before brazing heating was 260 MPa or less and the formability was excellent, the tensile strength after brazing heating was 170 MPa or more and the strength was high, the spontaneous potential after brazing heating was −700 mV to −780 mV and a suitable self corrosion resistance and sacrificial anode effect were shown, and the buckling load (average value) was 290N or more and excellent buckling resistance was exhibited.

The fin material of Comparative Example 1 (Alloy No. 4) has too low a content of Si, so the tensile strength after brazing heating was less than 170 MPa and the strength was insufficient, the buckling load (average value) was less than 290N, and the buckling resistance was inferior.

The fin material of Comparative Example 2 (Alloy No. 5) was too low in contents of Si and Mn, so the tensile strength after brazing heating was less than 170 MPa and the strength was insufficient, the spontaneous potential was less than −780 mV, and the buckling load (average value) was less than 290N and the buckling resistance was inferior.

The fin material of Comparative Example 3 (Alloy No. 6) was high in content of Fe and was too low in contents of Si and Mn, so the tensile strength after brazing heating was less than 170 MPa and the strength was insufficient and the buckling load (average value) was less than 290N and the buckling resistance was inferior.

The fin material of Comparative Example 4 (Alloy No. 7) was high in content of Fe and was too low in contents of Si and Mn, so the tensile strength after brazing heating was less than 170 MPa and the strength was insufficient and buckling load (average value) was less than 290N and the buckling resistance was inferior.

The fin material of Comparative Example 5 (Alloy No. 8) was too high in Cu content, but was too low in Si content, so the tensile strength before brazing heating was over 260 MPa and the tensile strength after brazing heating was less than 170 MPa and the strength was insufficient.

The fin material of Comparative Example 6 (Alloy No. 9) was too low in Mn content, so the solidus temperature was less than 615° C., the tensile strength after brazing heating was less than 170 MPa and the strength was insufficient, the buckling load (average value) was less than 290N, and the buckling resistance was inferior.

The fin material of Comparative Example 7 (Alloy No. 10) was too low in Si and Mn contents, so the tensile strength after brazing heating was less than 170 MPa and the strength was insufficient, the buckling load (average value) was less than 290N, and the buckling resistance was inferior.

The fin material of Comparative Example 8 (Alloy No. 11) was high in content of Fe and too low in contents of Si and Mn, so the tensile strength after brazing heating was less than 170 MPa and the strength was insufficient, the buckling load (average value) was less than 290N, and the buckling resistance was inferior.

Example B

In this Example B, the results of study of the conditions of the primary process annealing, the conditions of the secondary process annealing, and other sheet manufacturing conditions in the second aspect of the invention of the present application will be explained.

Shaved slabs formed in Example A (Alloy No. 1, thickness 10 mm) were cold rolled in a first stage to sheet thicknesses of 4.0 mm, were raised in temperature in an annealing furnace by a speed of temperature rise of 50° C./hr, were held at 280, 400, 500° C.×2 hr, then were air cooled as primary process annealing treatment. Further, they were cold rolled in a second stage to sheet thicknesses of 0.083, 0.091, 0.110 mm, were raised in temperature in an annealing furnace by a speed of temperature rise of 50° C./hr and held at 150, 250, 300, 450° C.×2 hr, then were air cooled as secondary process annealing treatment. Further, the sheets were cold rolled by final cold rolling rates of 40% and 45% to fin materials with a final sheet thicknesses of 50 and 60 μm (tempering symbol: H₁ ₄).

The four levels of fin materials obtained (Comparative Examples 9, 10, 11, and 12) were, envisioning the conditions of actual brazing heating, raised in temperature from room temperature to near 600° C. by an average speed of temperature rise of 50° C./min, were held near 600° C. for 2 minutes or so, then were cooled by an average cooling speed of 100° C./min as heat treatment.

The obtained test materials were, in the same way as Example A, measured for tensile strength (MPa), spontaneous potential (mV), and other properties. The obtained results of measurement of the properties are shown in Table 3 together with the process annealing conditions and other sheet manufacturing conditions.

TABLE 3 Properties of Test Materials (Study of Process Annealing Conditions) Sheet manufacturing conditions Before After Primary Primary Secondary Secondary Final brazing brazing intermediate intermediate process process Final cold heating heating sheet annealing sheet annealing sheet rolling Tensile Tensile Spontaneous Alloy thickness temp. thickness temp. thickness rate strength strength potential no. (mm) (° C.) (mm) (° C.) (μm) (%) (MPa) (MPa) (mV) Ex. 1 1 4.0 400 0.083 300 50 40 244 173 −731 Comp. Ex. 9 1 4.0 500 0.091 300 50 45 211 165 −721 Comp. Ex. 10 1 4.0 280 0.110 250 60 45 282 177 −726 Comp. Ex. 11 1 4.0 400 0.083 450 50 40 229 161 −723 Comp. Ex. 12 1 4.0 400 0.110 150 60 45 266 178 −725 Note) Underlined values show values outside prescribed range of present application.

The fin material of Example 1 (Alloy No. 1) had a chemical composition within the scope of the present invention and was manufactured under sheet manufacturing conditions within the scope of the present invention, so the solidus temperature was 615° C. or more and the brazeability (erosion resistance) was excellent, the tensile strength before brazing heating was 260 MPa or less and the formability was excellent, the tensile strength after brazing heating was 170 MPa or more and the strength was high, and the spontaneous potential after brazing heating was −700 mV to −780 mV and a suitable self corrosion resistance and sacrificial anode effect were exhibited.

The fin material of Comparative Example 9 (Alloy No. 1) was within the scope of composition of the present application, but the primary process annealing temperature was 500° C. or too high, so the tensile strength after brazing heating was less than 170 MPa and the strength was insufficient.

The fin material of Comparative Example 10 (Alloy No. 1) was within the scope of composition of the present application, but the primary process annealing temperature was 280° C. or too low, so the tensile strength before brazing heating was over 260 MPa and the material was judged to be inferior in formability.

The fin material of Comparative Example 11 (Alloy No. 1) was within the scope of composition of the present application, but the secondary process annealing temperature was 450° C. or too high, so the tensile strength after brazing heating was less than 170 MPa and the strength was insufficient.

The fin material of Comparative Example 12 (Alloy No. 1) was within the scope of composition of the present application, but the secondary process annealing temperature was 150° C. or too low, so the tensile strength before brazing heating was over 260 MPa and the material was judged to be inferior in formability.

From the above, it can be understood that an aluminum alloy fin material for heat exchanger use having a chemical composition within the scope of the present invention and manufactured under sheet manufacturing conditions within the scope of the present invention has a tensile strength of 260 MPa or less, a solidus temperature of 615° C. or more, a tensile strength measured after further brazing heating and cooling of 170 MPa or more, and a spontaneous potential of −780 mV to −700 mV, so even if reduced to a final sheet thickness of 30 to 80 μm, is excellent in formability at the time of corrugation, has a suitable strength before brazing enabling easy fin formation, exhibits a suitable self corrosion resistance and sacrificial anode effect, and is high in strength and excellent in buckling resistance as a material for forming a working fluid passage. 

1. An aluminum alloy fin material for heat exchanger use excellent in buckling resistance containing, by mass %, Si: 1.3% to 1.6%, Fe: 0.30% to 0.70%, Mn: 1.8% to 2.3%, Zn: 0.5% to 2.0%, and Ti: 0.002% to 0.10%, further limiting, as impurities, Mg to 0.05% or less and Cu to 0.06% or less, and having a balance of unavoidable impurities and Al, where a final sheet thickness is 30 to 80 μm, a tensile strength is 260 MPa or less, a solidus temperature is 615° C. or more, and further a tensile strength when measured after brazing heating then cooling, is 170 MPa or more and a spontaneous potential is −780 mV to −700 mV.
 2. A method for manufacturing an aluminum alloy fin material for heat exchanger use excellent in buckling resistance comprising a continuous casting step of pouring a melt of the above composition described in claim 1 and using a twin-belt casting machine to continuously cast then take up in a coil a thickness 6 to 15 mm slab, a primary cold rolling step of cold rolling to a sheet thickness of 1.0 to 6.0 mm, a primary process annealing step of process annealing at 360 to 460° C., a secondary cold rolling step of cold rolling to a sheet thickness of 0.05 to 0.12 mm, a secondary process annealing step of process annealing at 200 to 350° C., and a final cold rolling step of cold rolling by a cold rolling rate of 20 to 50% to a final sheet thickness of 30 to 80 μm. 